1. Field of the Invention
The present invention relates to data communications, and more particularly, to detection of transmission paths compatible with digital modulation.
2. Description of the Related Art
Much of the public switched telecommunications network (PSTN) is implemented using digital data transport. Nonetheless, significant portions of the PSTN are still based on analog technology. For example, the xe2x80x9clocal loopxe2x80x9d portion of PSTN that connects a telephone subscriber to a central office (CO) is typically an analog loop. Additionally, analog portions may exist at other points along a communications path, e.g., as an analog channel in an otherwise digital circuit.
The current generation of 56 Kbps modems (e.g., those based on K56flex(trademark) or x2(trademark) technology or conforming to ITU-T Recommendation V.90) no longer assume that both ends of a communications path may be analog and suffer impairment due to quantization noise introduced by analog-to-digital converters (ADCs). Instead, such modems assume that there is only one (1) analog portion in a downstream transmission path from a digitally connected server modem to a client modem connected to an analog local loop. This assumption is reasonable in areas where most Internet Service Providers (ISPs) and business customers are digitally connected to the network and allows data signaling rates of up to 56 Kbps in the downstream transmission path. K56flex is a trademark of Lucent Technologies Inc. and x2 is a trademark of 3Com Corporation.
Although a variety of similar designs are available, modems conforming to the ITU-T Recommendation V.90 are illustrative. See generally, ITU-T Recommendation V.90, A Digital Modem and Analogue Modem Pair for Use on the Public Switched Telephone Network (PSTN) at Data Signalling Rates of up to 56 000 Bit/S Downstream and up to 33 600 Bit/S Upstream (09/98), the entirety of which in incorporated by reference herein. Recommendation V.90 defines a method for signaling between a modem connected to an analog loop (the analog modem) and a modem connected to the digital trunk (the digital modem). Modems in accordance with Recommendation V.90 take advantage of this particular arrangement to increase the data signaling rate from the digital modem towards the analog modem. The quantization noise from a mu-law or A-law PCM converter typically limits the signal-to-noise ratio (SNR) to about 38 dB. However, the detrimental effect of quantization noise can be avoided in certain cases. If there are no analog-to-digital conversions in the downstream path from the digital V.90 modem to the analog modem, the PCM codes from the digital modem are converted to discrete analog voltage levels in the local CO and are sent to the analog modem via the analog local loop. The analog modem""s receiver then reconstructs the discrete network PCM codes from the analog signals received. Using current techniques, 56 Kbps signaling rates can be achieved.
There is no specific means provided in Recommendation V.90 by which the analog client modem is to decide whether PCM signaling can be supported by the downstream channel. Rather, the client modem must make an inference about the condition of the channel during the training process. For example, one technique apparently used by some modem manufacturers is to use the L1L2 signal which is transmitted as part of probing and ranging in phase 2 of startup procedures specified in Section 9.2 of Recommendation V.90 to estimate the high-end frequency response of the channel. A channel over which PCM operation is possible will typically exhibit less roll-off at frequencies near the upper edge of the voice band (0-4000 Hz) than will channels that cannot support PCM. There is no assurance, however, that good high-frequency response will correlate with good PCM operation, and poor high-frequency response by itself does not prelude PCM operation. Therefore, such methods assume a relationship between high-frequency response and PCM operation that is not valid in many practical cases. If the client modem fails accurately to assess the channel, then subsequent data exchange either will fail or will occur at less than optimal speeds.
Accordingly, an improved technique has been developed for reliably determining whether the transmission path between data communications equipment is free of analog-to-digital conversions. Such an analog-to-digital conversion may occur in a variety places in the transmission path and introduces quantization noise that is typically incompatible with desired data signaling schemes. For example, a quantization noise introducing conversion may occur in private branch exchange (PBX) equipment, within the public switched telecommunications network (PSTN) due to a particular routing via at least one analog channel, or due to the effect of multiplexing equipment. More generally, the techniques described herein are applicable to detection of error introducing signal conversions.
In an illustrative embodiment, an improved digital connection detection technique allows a modem coupled to an analog loop to select between receive modes, e.g., between V.90/PCM and V.34 modes. Implementation of such a selection is an essential part of the training procedure for modems that conform to ITU-T Recommendation V.90; however, the techniques described herein are also applicable to other data communications configurations and equipment. For example, the techniques are also suitable for detection of quantization noise introducing conversions in a transmission path between digital modems digitally connected to the digital trunk of the PSTN.
These and other suitable configurations will be better appreciated by persons of ordinary skill in the art based on the specification and claims that follow.